The present invention relates generally to edge emitters and more particularly to using an edge emitter as a line source.
For a number of years, the best light source for scanning a piece of document has been a fluorescent lamp. FIG. 1 shows such a tube fluorescent lamp. A large potential difference generated between the two electrodes at the ends of the tube breaks down a noble gas, such as argon, in the tube. Currents then conduct through the tube vaporizing and ionizing mercury droplets in the tube. When the mercury ions recombine after being excited, ultraviolet radiation is generated. The tube is coated by phosphors, which transform the incident ultraviolet radiation to visible light.
A fluorescent lamp is commonly used as the light source in an office document scanner because of its low cost relative to viable alternative light sources. However, the fluorescent lamp has some shortcomings when used for this purpose. Most notably, the fluorescent lamp is not a stable light source. It is an arc lamp, with light output highly dependent on the localized temperature dynamics of the arc, noble gas and vaporized mercury. Consequently, the light intensity from the lamp varies both spatially and temporally along the length of the lamp. Such variation degrades the accuracy of scanned images. Also, the fluorescent lamp should be warmed-up prior to use, as the heat generated from the arc has to vaporize and uniformly distribute the otherwise liquid drops of mercury. Moreover, the fluorescent lamp is quite bulky and should be shielded to protect the scanner sensor from heat and stray light.
The problems are intensified in a color scanner as shown in FIG. 2A. In such a scanner, one typically needs three different broadband illuminators as the source to cover the visible spectrum. To scan the color of an area, each illuminator sequentially shines onto it. Reflections from each illuminator are measured to reconstruct the color of the area.
Normally, fluorescent lamps are broadband devices. Typically, the phosphors in each lamp are selected to irradiate in the red, green or blue of the visible spectrum, so that the three lamps fully cover the visible spectrum. In a prior art embodiment, the three lamps are put into an optical system so that they all illuminate a common scan line on an object, and the reflected light is measured by a sensor. This system works, but may be inaccurate, wasteful and complicated because in addition to all the above-identified difficulties of fluorescent lamps, the phosphors in each lamp age at different rates. This can lead to color error. Also, as shown in FIG. 2A, the light generated by each lamp is not directional. In scanning, one is looking at specific areas. The light that is not pointed towards those areas is wasted. In fact, such wasted light power usually tends to generate unwanted heat. Thermal isolation may be required.
FIG. 2B shows another prior art method using a single white light fluorescent lamp as the source of a typical scanner. In this example, the reflected beam is split into different paths to be measured by sensors that are sensitive to different colors. The difference in sensitivity to different colors may be achieved by placing different filters over the sensors. This method again incurs the weaknesses of a fluorescent lamp.
Note that lasers or light-emitting-diodes (LEDs) are not very suitable as broadband illuminators. This is because both lasers and LEDs are inherently narrow-band devices. If the source is made up of a red, a green and a blue LED, color error may occur for an object area that is not primarily red or green or blue.
It should be apparent from the foregoing that there is still a need for a broadband light source that is stable, spatially and temporally uniform, rugged, efficient, compact, relatively easy to implement and requires no warmup period. Preferably, the source can illuminate in different colors.